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Vulnerable dolphins Two dolphin species in north-western Australia are vulnerable to local extinction because they rarely mingle with their own kind outside their immediate location, a new study has found.

The genetic analysis, published today in PLOS ONE, also identified one of the first cases of successful breeding between the two different species.

The Australian snubfin dolphin (Oracella heinsohni) and the Indo-Pacific humpback dolphin (Sousa chinensis) are both listed as 'near threatened' by the International Union for the Conservation of Nature.

But little is known about these dolphin populations, and this has hindered efforts to conserve them

"We were looking to get a bit more information about how vulnerable they could be, and a greater understanding to inform environmental impact assessments," says an author of the new study, Alex Brown, from the Murdoch University Cetacean Research Unit.

Genetic analysis

Brown and colleagues analysed genetic samples from two Western-Australian populations of each dolphin species, that were separated by more than 200 kilometres.

They used a plastic dart to take small samples of skin from the dolphins and then used nuclear an mitochondrial DNA markers to trace the degree of movement and breeding between the two populations of snubfin dolphins, and between the two humpback dolphins.

"We found there was a considerable differentiation between our different populations, so there was a limited gene flow and limited movement between the populations that we sampled," says Brown, a PhD candidate.

"This suggests that these species, although you might find them up the coast in a lot of different places, they're not freely moving up and down hundreds and hundreds of kilometres of coast."

Vulnerability

The researchers say snubfin and humpback dolphins have likely developed a preference for a local habitat and its fish populations, and because they are social animals, they naturally group together and remain in that habitat rather than spreading themselves along the coastline

The findings have implications for management of these two species in an area of the Western Australian coastline that has seen increasing development in recent years.

"The concerns are that [the dolphins] are very dependent on the near-shore environment, within 5-10 kilometres of shore and water up to 20 metres deep," says Brown.

"So with the port developments that are happening along the Pilbara and the potential for those in the western Kimberleys, you're looking at a lot of potential for changing that environment in which they occur,"

"There's a vulnerability to localised extinction because if you were to lose all the animals from one particular area, they wouldn't necessarily just repopulate from adjacent areas."

The researchers say that management plans should seek to maintain effective population size and gene flow.

Strange dolphin

During the study, the researchers discovered a strange looking female dolphin hanging around with other snubfins but with a low, triangular dorsal fin, size and colouring more characteristic of a humpback dolphin.

Genetic analysis showed it was a hybrid of a snubfin mother and humpback father - a phenomenon that had been suspected from earlier observations of snubfin and humpback matings.

"The humpbacks do seem to bully the snubfins at times so it's been on the cards, but it's interesting to see that it does actually work," Brown says.